Antennas for reception of electromagnetic signals may be represented as a resonant circuit which is tuned to the reception frequency, that is to say it resonates at the reception frequency. A simplified parallel resonant circuit, as is illustrated in FIG. 1, is formed by a capacitance 1 and an inductance 2. The resonant frequency fres of tuned circuits with inductances and capacitances is in general calculated using the formula:
                              f          res                =                  1                      2            ·            π            ·                                          L                ·                C                                                                        (        1        )            
If the capacitor 1 shown in FIG. 1 is visualized as being opened, this results in an opened tuned circuit with a predominantly electrical near field. The opened capacitor 1 is represented in FIG. 2 by its respective halves 1 and 1′.
If the capacitor 1 is retained, and an inductance 2 with a single turn is used, then this results in an opened tuned circuit with a predominantly magnetic near field. A tuned circuit such as this is illustrated in FIG. 3.
A tuned circuit such as that illustrated in FIG. 3 is frequently used for reception of electromagnetic waves and of signals which have been modulated onto them. In general, this antenna is also referred to as a frame antenna, in which case the conductor loop of the inductance 2 may also assume shapes other than those illustrated in the figure, for example a rectangle. The circumference of the single turn of the inductance 2 in this case typically corresponds to the wavelength of the signal to be received, or to half or one quarter of the wavelength.
FIG. 4 shows one known antenna arrangement with a conductor loop and capacitive outputting of the received signal. The received signal is output via coupling capacitances 3 and via a coupling transformer 4, which matches the impedance to the connecting line and produces an unbalanced signal from the balanced signal. The known antenna arrangement may also have a variable capacitance 1, so that the resonant frequency of the antenna circuit can be adjusted within a range.
FIG. 5 illustrates one known antenna circuit with a conductor loop in which the received signal is output inductively. An output loop 6 is provided for this purpose, and is connected via a coupling transformer 4 to a receiving circuit, which is not illustrated. The antenna arrangement illustrated in FIG. 5 may also have a variable capacitance 1, by means of which the resonant frequency can be adjusted within a range.
The variable capacitances 1 which are provided for the two antenna circuits illustrated in FIGS. 4 and 5 are normally formed by capacitance diodes. The capacitance of capacitance diodes can be varied by means of a control signal applied to them, typically a control voltage. In order to decouple any DC voltage that is used for control purposes from other circuit parts, coupling capacitors are often connected in series with the capacitance diode. When using capacitance diodes with a high capacitance variation ratio Cmax/Cmin it is possible to make the upper cut-off frequency of the tunable range twice as great as the lower cut-off frequency.
Antenna configurations are also known in which a number of frequency ranges are split between the respective antenna circuits. One receiver circuit, which is connected to the respective two or more antennas, selects the antenna which is suitable for the frequency range to be received. These antennas have a higher tuned circuit Q-factor, thus resulting in better antenna selectivity. One such antenna configuration is illustrated in FIG. 6. The figure shows a first resonant circuit comprising the capacitance 1 and the inductance 2, as well as a second resonant circuit comprising the capacitance 1′ and the inductance 2′ connected by means of coupling capacitors 3 to switches 7. The switches 7 connect a respectively selected resonant circuit to a transformer 4, which matches the balanced antenna output to an unbalanced input of a receiver, which is not shown.
Another switchable antenna configuration, which is shown in FIG. 7, has only a single conductor loop 2. Coils 8 which are inserted into the conductor loop result in an effectively larger circumference of the conductor loop 2 than the actual geometric circumference. The coils can be entirely or partially bridged by means of switches 10, such that it is possible to switch between two or more effective coil circumferences. Two or more effective coil circumferences can be switched by means of an appropriate arrangement of switches, which is not shown. The other elements of the antenna circuit correspond to those shown in FIG. 6.
Particularly in the case of portable appliances, however, the antenna sizes are restricted by the size of the appliances and their handling convenience. Furthermore, in the case of both portable and stationary appliances, the reception situation varies continuously and quickly. This is due, inter alia, to the fact that objects or people in the vicinity of the antenna act like capacitances, which influence the tuning of the antenna. Owing to the very low antenna gain, broadband antennas are particularly disadvantageous in portable receivers, since the antenna geometry and the frequencies to be received are unfavourably related to one another.
It is thus desirable to produce an antenna circuit for a wide frequency range, which detects changes in the reception conditions and matches the antenna matching to the changed reception conditions.